Tire Size Box

255/45R20 vs 255/50R20

255/50R20 compared with 255/45R20

+3.5% diameter, 0.0" width

255/50R20 is 30.04" tall vs 29.04". More than the 3% rule of thumb, so check carefully before switching.

29.0"20" rim4.5"10.0" wide
255/45R20 (solid) compared with 255/50R20 (dashed outline), drawn to scale

With 255/50R20 tires, when your speedometer says 60 mph you are really going

62.1 mph

255/50R20 is +1.00 inches (+3.5%) in overall diameter. That is more than the 3% most shops and makers recommend.

255/45R20255/50R20Difference%
Overall diameter29.04"30.04"+1.00"+3.5%
Section width10.04"10.04"0.00"0.0%
Sidewall height4.52"5.02"+0.50"+11.1%
Circumference91.22"94.37"+3.15"+3.5%
Revolutions per mile694.6671.4−23.2−3.3%

Speedometer difference

If your vehicle came with 255/45R20 and you fit 255/50R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.0 mph+1.0 mph
40 mph41.4 mph+1.4 mph
50 mph51.7 mph+1.7 mph
60 mph62.1 mph+2.1 mph
65 mph67.2 mph+2.2 mph
70 mph72.4 mph+2.4 mph
75 mph77.6 mph+2.6 mph
80 mph82.8 mph+2.8 mph

Each size on its own

Questions

Which is bigger, 255/45R20 or 255/50R20?
255/50R20 is taller by 1.00 inches (30.04" vs 29.04"). 255/45R20 is the same width.
Can I replace 255/45R20 with 255/50R20?
It is not recommended without checking carefully. The diameter difference is +3.5%, more than the 3% most shops and makers allow. It will throw off the speedometer and may affect ABS, stability control and clearance.
How much will 255/50R20 change my speedometer?
When your speedometer reads 60 mph with 255/50R20 fitted (calibrated for 255/45R20), your real speed is 62.1 mph. Your odometer will under-count miles by about 3.5%.
Will 255/50R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.50 inches of ground clearance at the axle.

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